The invention relates to conditionally stable operational amplifiers.
An operational amplifier is a relatively high gain amplifier capable of being used in various kinds of feedback circuits. An operational amplifier circuits can, for example, provide programmable gain, signal summation, integration, and differentiation, and various other useful functions.
The most popular variety of operational amplifier has high-impedance differential signal inputs and a low impedance signal output, and functions as a high-gain differential voltage amplifier. Another kind of operational amplifier, known as an xe2x80x9coperational transconductance amplifier,xe2x80x9d has high-impedance differential signal inputs and a high-impedance signal output, and functions as a differential voltage to current converter.
High accuracy operational amplifier circuits require large gain from zero frequency up to a certain closed-loop bandwidth. Most general-purpose operational amplifiers are constructed with a dominant pole in the open-loop frequency response in order to guarantee stability when any purely resistive voltage divider network provides a feedback signal. When the operational amplifier has such an open-loop frequency response, an enormous gain-bandwidth product is required for high accuracy. Designers of low-power or high-accuracy operational amplifier circuits have therefore considered conditional stability as a way of avoiding the gain-bandwidth product limitation of unconditionally stable operational amplifiers. A conditionally-stable operational amplifier has at least 180 degrees of phase lag for a frequency less than the frequency at which the operational amplifier has an open-loop unity gain, but the phase lag decreases to less than 180 degrees as the frequency increases to the open-loop unity gain frequency.
In accordance with a described embodiment, stability of a conditionally stable operational amplifier is ensured against large signals and transients (and associated saturation of the amplifier circuit) by use of a saturation detector and bypass circuits which collapses the system to an unconditionally stable system. The conditionally stable operational amplifier includes at least three integrator stages in a low frequency path from a signal input to a signal output, and a bypass path for bypassing at least one of the integrator stages in the maximum integration path. The bypass path(s) and the maximum integration path converge at a summing node combining signal from the low-frequency path with signal from the bypass path(s). According to this embodiment, stability of a conditionally stable multi-path operational amplifier is ensured against large signals and transients by disabling or bypassing low-bandwidth integrators when the output of the amplifier system is saturated.